The present invention relates in general to rotary regenerative air heaters for transferring waste heat from boiler flue gas to combustion air and, in particular, to a new and useful automatic mechanical cold end sealing system connected to air seals of a duct hood frame in the regenerative air heater.
In one type of rotary regenerative air heater, a cylindrical heat exchange mass and a containment structure, called the stator, are positioned stationary between the inlets and outlets of the air and gas ducts. The stator is a radially compartmented steel shell packed with a multiplicity of plates, arranged to provide an axial passage therethrough, such that the gas and air flow in an axial direction through the cylindrical heat exchange mass. The plates embody shapes, materials and thicknesses designed to provide optimum heat transfer, low pressure drop, resistance to corrosion, and ease of cleaning.
Air ducts at each axial end of the stator include air duct hoods, co-axially aligned with the cylindrical heat exchange mass, which are secured to a central drive shaft for co-axial rotation in relation to the cylindrical heat exchange mass. Each of the air duct hoods comprises a respective central flow inlet or outlet passage centrally mounted between, and in fluid communication with, two diametrically opposite hood segments or sectors for the passage of air to or from the heating mass. The hood segments of each of the air duct hoods are generally pie-shaped and circumferentially spaced from each other.
The gas ducts are arranged stationary, also at the opposite ends of the stator, and surround the rotating air duct hoods.
The air duct hoods, at upper and lower ends of the heat exchange mass, rotate synchronously so that alternate radial sectors of the mass of plates are alternately exposed to a hot flue gas stream and then cooled by a combustion air stream thereby effecting a regenerative heating and cooling cycle.
Sealing between the stationary and rotating components is achieved by articulated seal frames which are spring mounted to the rotating air duct hoods. As the stator expands or contracts, the frames adapted to the stator's periphery in order to maintain an effective seal at various boiler loads. The seal frame, which extends along the peripheries of the hood segments adjacent the heat exchange mass, carries a sealing strip. The sealing strip is resiliently urged in close proximity to cylindrical and radial end surfaces of the stator. During rotation of the air duct hood, the sealing strip slides along the end surfaces in sealing relationship with the stator.
An expansion joint is connected between the seal frame and the air duct hood to accommodate relative thermal displacement.
U.S. Pat. No. 4,669,531 discloses such a rotary regenerative air heater.
Apparatebau Rothemuhle of Germany manufactures this type of regenerative air heater.
In the Rothmuhle type regenerative air heater, sealing at the hood-stator interface is maintained by use of replaceable cast iron seal shoes mounted on a structural steel seal frame which is connected to the hood by a single fold expansion joint. The seal frame is supported on the hood by a number of spring bolt assemblies at the hood perimeter. Radial portions of the seal frame are equipped with a number of hinged articulation joints which enable the seal frame to follow the curvature of the stator as it deforms during operation.
Rothmuhle type air heaters employ thermally activated devices on the hot hoods to automatically lower seal shoes as the stator distorts, thereby maintaining minimum seal gaps. The thermal devices are not used on cold end hoods because temperatures are too low for them to function effectively. Therefore, seal gaps on the cold hood are preset away from the stator in the cold condition by the amount of expected maximum stator distortion which occurs at full load.
FIG. 1 illustrates the cold condition for a regenerative air heater generally designated 20, which comprises a stator 21 which has not yet been deformed due to thermal stresses, a hot hood 31 and an opposite cold hood 32. The known thermal activated devices 33 are shown in place on the hot hood. FIG. 1 also illustrates the substantial preset gap 34 which occurs between the lower surface of the stator 21 and the upper sealing surface of the cold hood 32, in the cold condition.
FIG. 2 corresponds to FIG. 1 but illustrates the heater 20 in its hot condition. In this condition, the stator 21 has been thermally distorted by a stator distortion amount 35 which serves to move the lower surface of the stator 21 into closer association with the upper sealing surface of cold hood 32.
Rothemuhle has also developed an automatic cold end seal system which utilizes an electro-mechanical device to maintain a programmable seal gap setting throughout the boiler's normal operating range to reduce air heater leakage. Inductive sensors placed in a flange of the air heater sense seal gap magnitudes as the hoods rotate. Signals are sent to a control computer where they are evaluated and compared to a preprogrammed gap. Adjustments are made by a motor driven ramp which activates a contact lever arm on the rim of the cold hood, to raise or lower the seal frame and bring the gap into tolerance. This device has the disadvantage of being complex. It also relies on the use of sensors in a hostile environment. Failure of any one of the critical components would also lead to failure of the seal.
Other solutions for adjusting the seal gap in a regenerative heater have also been proposed over the years.
U.S. Pat. No. 3,232,335 discloses pneumatic, mechanical and magnetic systems for sensing and thereafter compensating for variations in the seal gap. U.S. Pat. No. 3,246,686 discloses an arrangement which utilizes a plurality of individually rolling elements mounted independently of each other and connected to a floating sealing plate. The wheels roll against an annular flange of a rotor, and vary the force applied to the floating sealing plate as a function of the rolling action. U.S. Pat. No. 3,344,849 utilizes a spring plus counter weight arrangement for varying the force applied to a seal plate in a regenerative heater. All of these proposals suffer from complexity and sensitivity to mechanical, pneumatic or electrical failure.